Surface Plasmons

I'm still trying (yet unsuccessfully) to deal with surface plasmons, and I still hope on your help.
let me start like this:
I find in the review that:

We consider an interface in the xy-plane between two half-infinite spaces, 1 and 2, of materials the optical properties of which are described by their complex frequency-dependent dielectric functions [tex]\epsilon[/tex]1([tex]\omega[/tex]) and [tex]\epsilon[/tex]2([tex]\omega[/tex]), respectively. We ignore magnetic materials. Surface polaritons can only be excited at such an interface if the dielectric displacement [tex]\stackrel{\rightarrow}{D}[/tex] of the electromagnetic mode has a component normal to the surface which can induce a surface charge density [tex]\sigma[/tex],
(D2-D1)z=4[tex]\pi[/tex][tex]\sigma[/tex]

and here I found that I don't quite understand why there must be a component, normal to the surface, and what is the connection between surface charge density and surfacce plasmons.